Generally, percussion pistons comprise a piston in which one flange portion formed in the piston, is a means moving the piston to and fro by varying the pressure on either side of the flange. The pressure puts the piston into acceleration towards the drill rod and the piston is also returned to its initial position by pressure. From the bit of the drill rod, the impact energy is absorbed into rock that is being drilled. The required drilling force grows as a function of penetration, when the bit starts to penetrate from its initial position against the rock by impact force. If, with regard to time or penetration, the shape of the energy pulse transmitted to the drill rod does not correspond to an energy pulse shape which the rock being drilled can absorb, a portion of the energy pulse must, inconveniently, reflect back to the drill rod. The pulse shape transmitted to the drill rod on percussion, depends on the piston length and its sonic speed in the piston material. In previous designs no attention has been paid either to the shape of the onward pulse or to shaping it in the drill rod.